1. Field of the Invention
The present invention is in the field of electronic detector devices. More particularly, the present invention is in the field of infrared linear photoconductive detector devices which provide an electrical output signal indicative of infrared radiation falling on the detector. For example, such detectors may be used to provide an image of a scene illuminated with or radiating infrared light. Alternatively, such detectors may be used to provide an electrical signal indicative of a parameter or value of a process, or of the product of a process. Such a use might include monitoring the temperature at which a process is operating, or the temperature or other parameter of the product of a process at a particular location of the process. The degree to which the product transmits, reflects, absorbs, or otherwise alters infrared light at a particular location in a manufacturing process might also be used as an indicator of the correctness of the process, or quality level of the product as it is produced. Accordingly, such sensors may be used to monitor and correct a continuous process without the need for allowing product of the process to complete processing before measurements of the product can be taken.
2. Related Technology
Conventional infrared detectors are known which are responsive to different colors, or wavelengths, of infrared light. As used herein, different "color" is intended to denote different wavelength, or frequency, of light or electromagnetic radiation, whether or not this light may be viewed with the unaided human eye. Accordingly, differing "color" is used to mean differing wavelength of invisible infrared radiation or light also. Further to the above, conventional sensors are known which will provide a distinct output signal in response to infrared light in either one of at least two different wavelength bands. One conventional way of providing such conventional infrared detectors involves providing individual detector portions or elements of the detector, which detector portions themselves are responsive to different wavelengths of light. These conventional detectors include so called "diced" detector elements, each of which is fabricated from one of two different materials, such as PbSe and PbS. These materials have differing responses to infrared radiation having differing wavelengths. The conventional detectors of this type are referred to as "diced" because the detector elements themselves are fabricated on a substrate, and thereafter are cut or diced into individual detector element pieces, either from the substrate or along with a portion of the substrate. These diced detector elements are then combined with one another into a single detector structure. Adjacent detector rows of detector elements in the completed detector, for example, are fashioned of the differing materials having differing responses to infrared radiation of selected wavelengths.
Another conventional way of providing a detector having two or more differing response characteristics to infrared light having differing wavelengths is to provide a detector having an array of detector elements all fabricated of the same material, and also including a segmented filter structure interposed between the detector elements and the light source within the thermal housing of the detector. This segmented filter structure will filter the light reaching some of the detector elements so that these elements have a different response characteristic than the detector elements receiving unfiltered light. However, because such a detector requires a thermal housing to protect it from ambient temperatures and allow it to be chilled at least to near-cryogenic temperatures for best detection response to infrared light, and the segmented filter is disposed within this closed thermal housing, the filter choice made at the time the detector is manufactured is the only one ever available with a particular detector. That is, the filter sealed within the thermal housing can never be changed in order to optimize the detector to a particular operating environment or use without breaking the seal of the housing to gain access to the filter.
Still another conventional way of providing such a detector with two or more different response characteristics is to provide a detector array on which two or more optical band pass coatings (filters) are directly deposited onto the detector elements of the detector. This fabrication process usually requires two separate optical interference deposition processes, one for each of the filter coatings applied to the array. Again, the filters are sealed within a thermal housing and can never be changed. Also, because the filter material is applied directly to the detector itself as a semiconductor fabrication step or steps, the costs of fabrication of the detector are increased significantly. Also, the risk of detectors being unsatisfactory because of a defect in the filter application steps or steps statistically decreases the yield of good detectors which can be fabricated by this process.
All of the conventional methods of providing such detectors are expensive. Segmented filters cost from $1200 to $10,000 each at current fabrication costs. Diced detectors limit the use of the detector to wavelengths inherent in the material from which the detector is fabricated. Also, the cut-on and cut-off characteristics of detector materials themselves is not as sharp as can be obtained with an optical filter. All of the conventional detectors do not allow the wavelength response bands of the detector to be altered after the detector is completed. That is, the filters or detector material of the detector are set at the time of manufacture and can not thereafter be altered. All of these conventional detector structures are labor intensive to make, and require additional processing steps which increase the scrap rate of defective detectors. Accordingly, all of the conventional two-color detectors are costly. Also, these conventional detectors are limited to the colors selected for the materials of construction used during their manufacture. The color range for response of the conventional detectors can not be changed after they are manufactured.